(Reuters) - President Barack Obama's Democratic Party has to do more than just hold on to the White House in the 2012 elections.
Democrats have a slim majority in the 100-member U.S. Senate with 53 seats, including two held by independent who usually caucus with them. Almost half of these seats are up for election at the same time as the presidential election on November 6, 2012.
Republicans command a majority in the U.S. House of Representatives, whose 435 seats are also up for election next year.
A gain of four Senate seats would hand Republicans the majority. And only three would be enough if they also win the White House because a Republican vice president would have the power to cast tie-breaking votes in an evenly split Senate.
Republicans and Democrats face off in races for 33 Senate seats, 21 of them held by Democrats, 10 by Republicans and two by independents. Here's a list of nine races that could affect the balance.
Former governor Linda Lingle's bid for retiring Democratic Senator Daniel Akaka's seat has turned Obama's home turf into an unlikely Senate battleground.
Lingle, Hawaii's first Republican governor in almost 50 years, has appeal among moderate and independent voters. She enjoyed high approval ratings and easily won her second term with 63 percent of the vote in 2006.
The election would likely pit Lingle against Democratic Representative Mazie Hirono who has the backing of the state's senior senator, Daniel Inouye, a Democrat. In 2002, Lingle beat Hirono for the governorship by 5 percentage points. Another candidate is Democrat Ed Case, a former U.S. representative who unsuccessfully challenged Akaka in 2006.
Republicans are eager to hold on to the Senate seat Scott Brown won over a Democratic candidate after the death of longtime Democratic Senator Edward Kennedy in 2009.
Democrats are optimistic that Elizabeth Warren, a prominent consumer advocate and Harvard professor known for locking horns with the Wall Street, can unseat Brown. Warren played a key role in creating the Consumer Financial Protection Bureau and was chairwoman of the congressional panel that oversaw the U.S. banking bailout in 2008.
Brown is the first Republican in a generation to win a Senate seat from Massachusetts, a state with an independent streak. He has been popular in the state, and has staked out mostly moderate positions in the Senate.
Claire McCaskill, a freshman Democrat, faces an uphill battle to hold on to her seat in a Republican-leaning state.
This year, she has come under fire for using almost $90,000 in federal money for flights on her private plane and failing to pay $300,000 in local taxes.
Her Republican rivals have their own problems. Sarah Steelman, a former state treasurer, had to self-fund $400,000 to stay afloat and Representative Todd Akin faces a possible violation of state election laws that would make him ineligible for office.
John Brunner, a businessman who chairs a cosmetics and healthcare company, has also entered the Republican race but is relatively unknown in political circles.
Jon Tester's election to the Senate in 2006 was hailed as the debut of an untapped political base, the "New Western Democrat" -- small-town, rural voters whose values often do not fit neatly with either party.
Now he faces a tough re-election bid, reflecting troubles the Democratic Party may face in the West. Polls show Tester and his principal rival for the seat, Republican Representative Denny Rehberg, at a draw for the past three months.
Both want to paint themselves as the more populist candidate. Rehberg accuses Tester of pandering to Wall Street interests and falling in lock-step with Obama, while Tester says Rehberg, a rancher and real-estate developer, is anti-environment and misleading about his wealth.
Ben Nelson is widely seen as one of the most vulnerable incumbent Democrats in the 2012 election.
Nelson, considered a centrist, is under fire for supporting Obama's healthcare overhaul in a state where Republicans account for 48 percent of all registered voters, according the Cook Political Report.
Jon Bruning, the state attorney general, was the early Republican favorite, but some question his electability because of his abrasive style and gaffes, including comparing welfare recipients to raccoons.
Other Republican candidates include State Treasurer Don Stenberg, who has lost three earlier Senate bids, and Deb Fischer, a state senator who represents a Republican stronghold in the western part of Nebraska.
Republican Senator Dean Heller will defend his seat against Democratic Representative Shelley Berkley.
Heller is a former U.S. House member who was appointed to serve the rest of fellow Republican Jon Ensign's Senate term after Ensign was snared in a sex scandal and resigned earlier this year.
In a state with one of the highest levels of unemployment and housing foreclosures, Heller, a former secretary of state for Nevada, will market his credentials as a fiscal conservative.
The race could depend on how much each candidate can make inroads outside their base. Berkley has solid backing in her Democratic district which includes Las Vegas and is home to almost 70 percent of Nevadans. Heller's base is in the northern, more rural parts of the state.
Republicans strategists say the party may be able to win in a solidly Democratic state because Democrats face a contentious primary to compete for retiring Democratic Senator Jeff Bingaman's seat.
Top Democratic contenders are two-term Representative Martin Heinrich and State Auditor Hector Balderas, who has surpassed campaign fundraising expectations. Balderas could appeal to the state's more than 40 percent Latino population.
On the Republican side, analysts consider former Representative Heather Wilson, a centrist, as the front-runner, ahead of Lieutenant Governor John Sanchez and businessman Greg Sowards, although Wilson's reputation as a moderate could cause problems for her in the primary race.
Republican George Allen and Democrat Tim Kaine are the likely contenders for retiring Democratic Senator Jim Webb's seat in a swing state Democrats are likely to have a tough time winning in the 2012 general election.
Allen is a U.S. senator who lost his seat to Webb in one of the biggest upsets of the 2006 election cycle. Allen was criticized during the campaign for using a racial slur for Indian-Americans. Kaine is a former Virginia governor and chairman of the Democratic National Committee.
Polls show the two nearly neck and neck -- Kaine leading by 1 percentage point in October and September.
The match-up for retiring Democratic Senator Herb Kohl's seat will likely be between Democratic Representative Tammy Baldwin and former Republican Governor Tommy Thompson.
Thompson, a former U.S. secretary of health and human services, is one of the most popular politicians in the state where he has served four terms as governor, but some conservatives are suspicious of him for his support of parts of Obama's healthcare overhaul. Other contenders for the Republican nomination are former Representative Mark Neumann and Jeff Fitzgerald, a former speaker of the Wisconsin state assembly.
Baldwin has won the endorsement of Russ Feingold, a former Democratic senator from Wisconsin. If elected, Baldwin would be the first openly gay U.S. Senator.
(Reporting by Lily Kuo in Washington; editing by Mohammad Zargham)